1. Field
The present invention relates generally to wireless communications, and more specifically, to transmission diversity systems.
2. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for remote subscribers. As used herein, the term “cellular” system encompasses systems using either cellular or personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
The telecommunication standards cited above are examples of only some of the various communications systems that can be implemented. Most of these systems are now configured to use a single antenna for forward link transmissions to a single remote station, but it is envisioned that multiple antennas will eventually be used for forward link transmissions to a single remote station. Multiple antennas provide transmit diversity (TD), which increases the quality of the forward link. When the quality of the forward link improves, less transmission power is required to convey information to a remote station. Conversely, more information can be conveyed using the same transmission power, and the overall data throughput of the link is increased. As used herein, the term “forward link” refers to transmissions from a base station to a remote station while the term “reverse link” refers to transmissions from a remote station to a base station.
In transmission diversity systems, antenna phase information and amplitude information are acquired by a remote station through a pilot channel transmitted from each antenna. One of the antennas is considered to be the primary antenna, whereas the other antennas are considered to be the auxiliary antennas. For illustrative ease only, the embodiments that follow will be described as two-antenna systems. The number of antennas can be extended without affecting the scope of the embodiments described herein.
A problem arises with the deployment of systems offering transmission diversity. Namely, while a communication system may be upgraded to utilize multiple antennas, the remote stations operated by individual users may not keep pace with the system upgrades. The current state of the art envisions a system that can support a non-TD remote station by using the primary antenna alone. Since almost all wireless communication systems require the transmission of characterizing information from the remote station to a serving base station, a TD base station can easily be notified that transmissions to a non-TD remote station should be sent only on the primary antenna. The modulation at the base station can then be altered accordingly.
However, the current method of transmitting only on the primary antenna to a non-TD remote station is flawed. The flaw arises from the deleterious effects of interference between antenna transmission paths when both a non-TD remote station and a TD remote station are operating in a system with transmission diversity. In spread spectrum communication systems, such as, e.g., CDMA and W-CDMA, orthogonal and quasi-orthogonal codes, such as the Walsh code sequences, are used to channelize the information sent to each remote station on the forward link. In other words, Walsh code sequences are used on the forward link to allow the system to overlay multiple users, each assigned a different orthogonal or quasi-orthogonal code, on the same frequency during the same time duration.
Hence, the signals originating from one antenna transmission are orthogonal and the magnitude of the interference between these orthogonal signals is correlated. However, even though the signals originating from multiple antennas may be orthogonal, the transmission medium will introduce imperfections such as multi-path, which will degrade the orthogonality between signals. Signals received by the remote stations will not be entirely orthogonal and will thereby interfere with each other. The magnitude of the interference between one antenna transmission path and another antenna transmission path is generally not correlated, since the signals from the different antennas propagate along different wireless paths. If the magnitude of the interference between the various antenna paths is not correlated, then the transmission gains arising from combining multipaths are no longer present for non-TD remote stations. A more detailed explanation of this phenomenon is presented below.
Since a base station with transmission diversity will be transmitting to both TD remote stations and non-TD remote stations during the same time duration and on the same frequencies, it follows that the performance of a non-TD remote station suffers greatly when operating amidst TD remote stations. Therefore, there is a need in the art for methods and apparatus that allow a non-TD remote station to operate within a transmission diversity system in a manner that does not impair the quality of the received forward link transmissions.
Besides the aforementioned problem suffered by non-TD remote stations within a TD system, another situation occurs that reduces the effectiveness of the TD system. In certain situations, available transmission capacity may be left unused due to a system inability to balance the power between multiple antennas. When the power load on the multiple antennas is unevenly distributed or when the power ratings on the power amplifiers for each antenna are different, current systems are not configured to redistribute the transmission load from one overburdened antenna to another under-utilized antenna.
Typically, each antenna has its own separate power amplifier. Power amplifiers are rated for a maximum power, based on design constraints and regulatory constraints. There is therefore a limit to the amount of power with which each antenna can transmit. In current systems, the system is designed to be limited whenever one of the power amplifiers reaches its maximum load, even if another power amplifier has capacity available. In other words, available power from an under-utilized antenna cannot be redistributed to a heavily loaded antenna. This problem may occur when multiple antennas are not loaded with the same transmission load, or when different power amplifiers have different power ratings.
There are certain situations wherein the multiple antennas are not loaded with the same transmission power. One situation is the case where there are transmissions to both non-TD and TD remote stations, such that transmissions to non-TD remote stations occur only on the primary antenna and transmissions to TD remote stations occur on both primary and auxiliary antennas. It is impossible to pre-determine the required power ratings of the primary and auxiliary power amplifiers on a base station supporting TD, since power depends on a fluctuating number of TD and non-TD remote stations that are serviced by the base station at any given time. The base station throughput capacity is therefore likely to be limited by the power amplifier of a fully loaded, primary antenna, since the power from the other power amplifiers cannot be “borrowed.” Note that the power from the other power amplifiers cannot be utilized because of legal constraints imposed by the various standards bodies, and not because of a physical constraint.
Another situation in which the power amplifiers have uneven loads may occur during the transmission of certain channels. For example, the paging channels and synchronization channels of cdma2000 are configured for transmission only on the primary antenna, which affects the load on the primary power amplifier. The increased load makes it more likely that the power amplifier for the primary antenna will reach full capacity before the power amplifiers for the auxiliary antennas, so that the power amplifiers on the auxiliary antennas will have available power that is left unused. This translates into wasted capacity. Therefore, there is also a need in the art for methods and apparatus that will increase the forward link capacity of TD systems by utilizing the “wasted capacity” of the auxiliary antennas that is caused by full loading on the primary antenna.